220 8650 article Path: news.gmane.org!not-for-mail From: Nevin Liber Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: string_view::is_null() Date: Fri, 17 Jan 2014 13:58:37 -0600 Lines: 107 Approved: news@gmane.org Message-ID: References: <8045a4d2-721d-4725-8bb7-7a91b6f53ec8@isocpp.org> <186E0927-C962-4E34-AED0-E7509DD1F0A0@gmail.com> <7aad44d8-6b4e-438a-b309-bd898b994b34@isocpp.org> <9C2C439F-7983-4224-B272-53CAD0E2E456@gmail.com> <3379bbae-dd54-461c-bf90-228e476ca7db@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=047d7bb04c442e701004f02ffa41 X-Trace: ger.gmane.org 1389988753 24702 80.91.229.3 (17 Jan 2014 19:59:13 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 17 Jan 2014 19:59:13 +0000 (UTC) To: "std-proposals@isocpp.org" Original-X-From: std-proposals+bncBCE35H5S6IDBBF4X42LAKGQEHVSFV7I@isocpp.org Fri Jan 17 20:59:21 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ee0-f71.google.com ([74.125.83.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W4FZY-0001tr-6A for gclcip-std-proposals@m.gmane.org; Fri, 17 Jan 2014 20:59:20 +0100 Original-Received: by mail-ee0-f71.google.com with SMTP id t10sf7761675eei.6 for ; Fri, 17 Jan 2014 11:59:19 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:sender:in-reply-to:references:from :date:message-id:subject:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=WvUpeA+IFXBW74Czsv7flZxBLbjLo7wrp3g3Q7Br1CI=; b=FA5uvAeNauFGn+7YoHaI6ZhUTA2g8sZpqLAH0XrFo6VDbA7K526UIsA/e3ph5BE4fQ 2Ne1V6nhC6hJpVywQE6CfXXLdjmZFTH1jKaXbUr5SbUpIY8uH/JAZ5/JHL7bAtnOKM/c X76cB+1z/oIRUCdDoUgSLlSd8dQ84zNE8p2e15mtrvJzGeKkS2eGtoJScTkTAOHhDqvL cLnEUd6ljK2+uOz/R6M4xu2SNlsLp4R6Ari8JlGhY3NGwyyw1QhfO61Xvqzb6XahBxvV eAN/nBkhshwZQqUONljE3nqYN3PaISDVHw72gQT2pImZ2YNxgOL9sa5z7z0MXPF2VWy4 t2iA== X-Gm-Message-State: ALoCoQm4Lq+BIRPjpL1w9QEA0v4zcFnLvNz5+/IQzEJHkrclJF2hwMLN5XCmru5JcX6/mJE0dotS X-Received: by 10.152.27.73 with SMTP id r9mr1804451lag.4.1389988759756; Fri, 17 Jan 2014 11:59:19 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.89.2 with SMTP id bk2ls175779wib.13.canary; Fri, 17 Jan 2014 11:59:18 -0800 (PST) X-Received: by 10.180.94.7 with SMTP id cy7mr27444wib.21.1389988758454; Fri, 17 Jan 2014 11:59:18 -0800 (PST) Original-Received: from mail-wi0-x234.google.com (mail-wi0-x234.google.com [2a00:1450:400c:c05::234]) by mx.google.com with ESMTPS id mw18si2164654wic.8.2014.01.17.11.59.18 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 17 Jan 2014 11:59:18 -0800 (PST) Received-SPF: pass (google.com: domain of nliber@gmail.com designates 2a00:1450:400c:c05::234 as permitted sender) client-ip=2a00:1450:400c:c05::234; Original-Received: by mail-wi0-f180.google.com with SMTP id d13so1177552wiw.1 for ; Fri, 17 Jan 2014 11:59:18 -0800 (PST) X-Received: by 10.195.13.130 with SMTP id ey2mr3739021wjd.7.1389988758317; Fri, 17 Jan 2014 11:59:18 -0800 (PST) Original-Sender: nliber@gmail.com Original-Received: by 10.194.121.163 with HTTP; Fri, 17 Jan 2014 11:58:37 -0800 (PST) In-Reply-To: <3379bbae-dd54-461c-bf90-228e476ca7db@isocpp.org> X-Original-Sender: nevin@eviloverlord.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of nliber@gmail.com designates 2a00:1450:400c:c05::234 as permitted sender) smtp.mail=nliber@gmail.com; dkim=pass header.i=@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:8650 Archived-At: --047d7bb04c442e701004f02ffa41 Content-Type: text/plain; charset=ISO-8859-1 On 17 January 2014 12:34, Peter Bigot wrote: > > While the half-open range argument is true in isolation, it's missing the > point that the value of data() is well-known based on how the string_view > was created and what operations have been performed on it. > How does the callee know how the string_view was created? If the callee has more preconditions than those enforced by string_view, you might be better off using a separate type. If the callee "knows" how it was created, why use a string_view at all? > Adjusting the string_view to the point where size() is zero does not > invalidate data(). > > Based on how sv2 is constructed in that example, I believe it's guaranteed > that deref(sv2) will return the '\0' that terminates the string literal > used to initialize sv2, and that doing so does not invoke undefined > behavior. > What compilers are allowed to assume from the specification of the standard library is an interesting question... :-) Take for example: std::vector v(1); auto p1 = v.data(); v.pop_back(); Can I legally do a placement new of a std::string into the space pointed to by p1? Beats me. -- Nevin ":-)" Liber (847) 691-1404 -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. --047d7bb04c442e701004f02ffa41 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
On 17 January 2014 12:34, Peter Bigot &l= t;bigotp@acm.org>= ; wrote:

While the half-open range argument is true in iso= lation, it's missing the point that the value of data() is well-known b= ased on how the string_view was created and what operations have been perfo= rmed on it.

How does the callee know how the str= ing_view was created?=A0 If the callee has more preconditions than those en= forced by string_view, you might be better off using a separate type.=A0 If= the callee "knows" how it was created, why use a string_view at = all?
=A0
A= djusting the string_view to the point where size() is zero does not invalid= ate data().

Based on how sv2 is constructed in that example, I believe it's gua= ranteed that deref(sv2) will return the '\0' that terminates the st= ring literal used to initialize sv2, and that doing so does not invoke unde= fined behavior.=A0

What compilers are allowed to assume= from the specification of the standard library is an interesting question.= ... :-)

Take for example:

std::vector<= ;std::string> v(1);
auto p1 =3D v.data();
v.pop_back();

<= div>Can I legally do a placement new of a std::string into the space pointe= d to by p1?=A0 Beats me.
--
=A0Nevin ":-)" Liber= =A0 <mailto:= nevin@eviloverlord.com>=A0 (847) 691-1404

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